The need exists in a wide variety of industries for electrically conductive materials which can provide an object with a conductive surface or a conductive internal layer. A material capable of electrostatic dissipation, for instance, is desired for use in such disparate products as carpet backing, furniture intended for computer or electronics use, flammable chemical storage tanks, filtration media, and electrical component packaging. Thin conductive substrates also serve as diagnostic layers in composites or storage tanks, and may be used in products utilizing resistance heating, such as pipe wrapping, food warmers, or heated socks and gloves. And these materials see great use for electromagnetic interference (EMI) shielding in electronics cabinets, cable and wire shielding, and various aspects of the defense and aerospace industries.
While conductive materials have long been sought for these numerous applications, their use has been limited by cost and workability. Clearly, items such as carpet, computer furniture, and heated socks and gloves cannot utilize conductive layers when the production or incorporation costs of the layers push the price beyond reasonable limits. Thus, inexpensive, highly-workable conductive materials are strongly desired. Unfortunately, the materials currently in use are expensive to produce, difficult to work with, or both expensive and unworkable. For instance, graphite fibers and fabrics are expensive, have low flexibility, encounter dust and contamination problems, and are difficult to incorporate in structural materials.
Carbonized paper has a low permeability for any desired resins, is expensive, and has low flexibility and tensile/tear strength. Metal screens and fibers are expensive, have low flexibility, are difficult to work with, and react with resins. Conductive paints and lacquers are also expensive, require surface preparation of the material to be covered in addition to post-application drying and curing steps, may be difficult to apply, and are disfavored due to overspraying, waste, and the emission of volatile organic compounds. Vacuum metallized substrates also suffer from high cost and additionally degrade when a resin is employed. Carbon-polymer composites formed of extruded carbon fibers sheathed or cored with fabrics such as nylon or PET offer good properties but are expensive to produce. Synthetic metal-salt dyed fibers similarly suffer from a high cost.
Thus, the need still exists for a low-cost, workable conductive material which can provide a conductive layer to a wide variety of products.
One product of interest is in the area of reinforced resin parts having conductive properties. Fiberglass reinforced plastic (FRP) structural parts have been successfully used in various applications where the part is subjected to corrosive decay, decomposition, rust, and degradation, such as in chemical plants, paper mills, and plating facilities.
FRP parts or articles can be made by a number of processes, including, but not limited to, the following processes. They typically involve one of three types of cures--room temperature, elevated temperature, or ultraviolet ("UV").
Contact molding or open molding is an FRP process utilizing a room temperature cure. Resins and reinforcements are manually (hand lay-up) or mechanically (spray-up) deposited on an open mold surface. The mold surface is preferably previously coated with a gel coat and is provided with a surfacing fabric such as a mat or veil. Once the required amounts of reinforcements and resin have been deposited on the mold, the laminate is worked with rollers, brushes or squeegees, usually manually, to remove any trapped air and thoroughly saturate or wet-out the reinforcements with resin. Once this is completed, the laminate is allowed to cure at room temperature.
Resin transfer molding (RTM) and structural reaction injunction molding (S-RIM) are two similar closed mold FRP processes in which the required reinforcement package, including a surfacing fabric such as a mat or veil, is placed on one-half of the mold cavity, usually the bottom half. Once properly positioned, the top half of the mold is closed on the bottom half and secured in place. Next, the resin is injected slowly under minimal (e.g., 50 psi) pressure in RTM or rapidly under high pressure (e.g., 2000 psi) in S-RIM. The mechanical pumping and resulting pressure cause the air to be flushed out of the mold cavity and the resin to saturated or wet-out the reinforcement. The resin impregnated reinforced article is then allowed to cure at room temperature.
Compression molding is another FRP molding process. In this process, the reinforcement package including surfacing fabric (mat or veil) and the resin are placed on one-half, usually the bottom half, of the mold cavity. Once properly positioned, the top half of the mold is mechanically closed on the bottom half using a press which compresses the reinforcement package and resin under pressure (from 50 to 1500 psi) to flush out the air and thoroughly saturate or wet-out the reinforcement package with resin. It is then cured normally with the assistance of heat, i.e., elevated temperature cure.
Filament winding is an FRP process in which reinforcements, normally continuous rovings, are saturated with resin, normally by pulling them through a pan or bath containing the resin. The reinforcements are then wound on a rotating mandrel in a specific pattern. The mandrel may or may not have been previously covered with a resin impregnated surfacing fabric. One or more outer layers of surfacing fabric may be wrapped over the resin impregnated reinforcement when required. Once the required amount of resin, reinforcements and surfacing fabrics are properly placed on the mandrel, the laminate is allowed to cure either with or without the assistance of heat.
The continuous panel process is an FRP process for making continuous flat and/or shaped, e.g. corrugated, panels. It involves depositing a resin on a carrier film which then passes under a reinforcement deposition area. Various types of reinforcement are then applied to the film or resin. The reinforcement and resin then go through a compaction section where a series of belts, screens, or rollers force air out and thoroughly saturate or wet-out the reinforcement with resin. A surfacing fabric such as a mat or veil may be placed on either the top or bottom surface of the resulting saturated material and the fabric is allowed to be saturated with resin. A carrier film is then applied to the top surface of the resulting article which is passed through a curing station where the resin is normally cured with the assistance of heat. Once cured, the carrier film is removed and the article is cut to the desired length.
Pultrusion is a process for fabricating a reinforced resin product, such as a fiber reinforced plastic (FRP) article. It involves taking various forms of fiber reinforcements (mats, woven products, continuous rovings, etc.) made of materials such as fiberglass, carbon, aramid, etc. which are saturated or wet-out with an uncured thermoset resin. Normally, a polyester resin is used, but it can also be epoxy, phenolic or other resins. These saturated reinforcements are then pulled through a heated, matched metal die or mold machined to the shape of the desired finished part. While in the die or mold, the time and temperature relationship of the die or mold to the resin formulation transforms the resin from a liquid to a solid. This transformation is known as curing, cross-linking or polymerization. During this transformation, exothermic energy is generated in the chemical reaction.
In the pultrusion process, the amount and type of reinforcement needed to obtain the desired product is first determined. The reinforcement is put in the proper position and held in that position so that a uniform distribution of reinforcement in the resin is achieved. This is accomplished by using the proper amount of tension on the reinforcement along with guiding the reinforcement. If the reinforcement is not uniformly distributed throughout the cross-section of the resin, the finished product could possess areas of structural weakness.
Next, the reinforcements must be saturated or wet-out with a resin in, e.g., the resin tank. Preferably, all of the reinforcements must be wet-out to insure that a quality product is obtained. Viscosity, residence time, and mechanical action are all variables which influence the wet-out process. Without uniform and adequate wet-out, certain areas of the product may be structurally deficient.
Preformers can be used to manipulate the combination of reinforcement and resin in order to reduce die wear and insure uniformity. The combination of reinforcement and resin is then pulled through heated steel dies. Curing occurs during this step of the pultrusion process. Die temperature, pull speed, and the type of catalysts and cure promoter are all variables which control the rate of curing during formation of the product in the dies. The pull speed remains constant during the pultrusion process. Different shapes will require different speed settings.
The finished pultrusion product is then cut to the desired size. The resulting product possesses outstanding strength to weight ratio.
A publication of Fiberglass Canada, Inc. entitled "An Introduction to Fiberglass-Reinforced Plastics/Composites" provides a detailed overview of FRP production. The teachings of this publication are hereby incorporated by reference into this application.
Resin reactivity in FRP processes is controlled by a wide variety of properties. The base resin, as supplied by the resin manufacturer, will vary in reactivity based on formulation, viscosity, temperature in storage, age, etc. The curing of a resin is based on cross-linking the individual molecules to form long chain molecules. Cross-linking can be achieved by the use of a catalyst and/or heat. The rate of cross-linking is determined by how fast the catalyst disassociates into free radicals of active oxygen, which initiate the cross-linking.
The catalyst's rate of disassociation into free radicals can be controlled by heat and/or cure promoters. Hot-molding processes often do not require a promoter due to the high temperatures. It is known in the art to reduce curing time and/or heating requirements for the manufacture of resin materials by adding a promoter to the resin and curing agent. More heat and/or promoter results in more free radicals, faster cross-linking, faster cure and higher exotherm temperatures. This produces faster cure and faster line speeds. Too high an exotherm temperature, however, causes the finished part to be structurally weaker.
However, the graphite or carbon additives that can provide conductivity to a fabric or veil, and thus to the composite matrix, tend to inhibit cure due to the high surface activity of carbon (especially activated carbon). This is particularly the case for room temperature cures. Powders and dispersions of carbon are known to inhibit room temperature resin cure systems where free radical initiators such as benzyl peroxide, methyl ethyl ketone peroxide are employed. This is true for resins in general and for vinyl ester resins, in particular. The carbon absorbs the free radical, thereby negating its cross-linking effect on the composite matrix.
Thus, it is necessary to address the need for fast and efficient curing, especially at room temperature, of a molded resin body which has a conductive fabric or veil.